Isolated capsular muscle from cat spleen did not contract on exposure to a potassium-free bathing fluid. After exposure to conditions favouring downhill sodium and potassium movement (i.e. stimulation by noradrenaline in the absence of external potassium), subsequent reactivation of the sodium pump inhibited responses to noradrenaline. Under these special conditions, but not normally, the sodium pump may be behaving in an electrogenic manner.